I thought that Media Encoder took forever to export my stuff... then I realized it had continued waaaaaaaaay beyond the actual size of the composition (as determined by the bar in Ae), and continued for the full duration which was much longer than that bar. Since trying to tell it to use only the bar in Media Encoder is a huge chore, can I somehow easily "cut to the bar" in Ae with a couple of clicks or so?

In order to make the composition LONGER, you apparently *must* edit the compo settings and manually enter a value. However, I'm hoping there is an easy way to cut it down to only the length it actually turns out to be when you are done. It would solve this problem and also make it more pleasant to navigate.

Go to the out point of the longest layer of your comp. Hit the n (for eNd) key to define the end of the AE Work Area.

hit the Home key to go to the beginning of the comp. Hit the b (for Begin) key to define the beginning of the AE Work Area.

Now navigate to Composition>Trim Comp To Work Area.

Your comp is now as long as your longest layer. Need it shorter? Go to the point on the AE timeline when you want the comp to end, and repeat the process described above for setting the end of th Work Area, and trimming the comp to the work area.